There exist a large number of threats to computers connected to the Internet. A denial-of-service (DoS) attack one example of such threats. A DoS attack is an attack on a computer system aimed at causing it to crash, i.e., to create conditions under which legitimate (rightful) system users cannot gain access to the resources (servers) provided by the system, or to make this access difficult. The motives for these attacks may vary—they may be elements of a competitive struggle, a means of fraud or retaliation, an expression of dissatisfaction, a demonstration of capabilities, or an attempt to attract attention, which is most often interpreted as cyber terrorism. An attack that is carried out simultaneously on a larger number of computers is called a Distributed Denial-of-Service (DDoS) attack. There are two kinds of DDoS attacks: bandwidth attacks and application attacks.
Bandwidth attacks are characterized by flooding of network communication channels with a large number of packets. Since network routers, servers and firewalls have limited processing resources, they may become inaccessible for processing legitimate transactions or crash under the overload during DDoS bandwidth attack, in which a large number of TCP, UDP or ICMP packets is sent to a specific network device or service.
Application attacks are characterized by exploitation of features of TCP, HTTP and other protocols, and the behavior of services and applications, to capture the computing resources of the computer on which the target of the attack is working and thus prevent the latter from processing legitimate transactions and requests. Examples of application attacks are attacks on half-open HTTP connections and with false HTTP connections.
One popular technique for detection and prevention of DDoS attacks is to identify computers from which the attacks perpetrated and block all traffic from those computers. One shortcoming of this technique is that incorrect identification of such computers, i.e., false positives, may result in blockage of traffic from legitimate computers. For example, false positives can disrupt the operation of an online store by preventing regular visitors from placing orders. Accordingly, there is a need for a network security system that can reduce false positives during detection of DoS and other attacks on a protected computer system.